1. Field of the Invention
The present invention relates to optical interference films. The optical interference film is generally used with a lamp. Two different kinds of optical interference films are practically used. One of the optical interference films reflects infrared rays and transmits visible rays. The other optical interference film reflects visible rays and transmits infrared rays.
2. Description of the Related Art
Halogen lamps which may reduce the amount of infrared rays in the light radiated therefrom are known in recent years. Halogen lamp includes a straight bulb in which a tungsten coil filament is disposed along the center line of the straight bulb. At least one surface of inner and outer surfaces of the bulb is provided with an optical interference film. The optical interference film transmits visible rays and reflects infrared rays in light radiated from the coil filament. Therefore, infrared rays reflected by the optical interference film returns toward the filament, and heats up the filament, resulting in a high luminous efficiency in halogen lamp.
The above-described optical interference film includes a high refractive index layer made of titanium oxide and the like and a low refractive index layer made of silica and the like alternately accumulated one to the other at ten to fifteen layers on the bulb. Interference of light occurs when light from the light source, e.g., filament, passes through the high and low refractive index layers. The light of a specific wavelength range may be transmitted or reflected by controlling thickness of each refractive index layer.
In such an optical interference film, reflection factor of the optical interference film increases, as number of high and low refractive index layers increase. However, cracks or inter-layer peeling of the optical interference film tend to occur. In particular, since the bulb temperature of the above-described halogen lamp is high during the operation, cracks or inter-layer peeling tend to occur in the optical interference film attached to the bulb of the halogen lamp. To avoid such cracks or peeling, number of high and low refractive index layers are reduced in the optical interference film of the halogen lamp. Therefore, a desirable optical characteristics may not be achieved in the above-described optical interference film of halogen lamp.
Another solution of cracks or inter-layer peeling of the optical interference film is disclosed in U.S. Pat. No. 4,701,663, issued to the same assignee, and entitled LAMP HAVING INTERFERENCE FILM, and U.S. patent application Ser. No. 925,388/87 of the same assignee, and entitled LAMP WITH OPTICAL INTERFERENCE FILM AND METHOD FOR MAKING THEREOF, corresponding to Japanese patent Laid-open Publication No. 105357/87. In the above-described prior art, a high refractive index layer includes at least one metal oxide, as a main element, selected from titanium oxide, tantalum oxide and zirconium oxide. A low refractive index layer includes silica, as a main element. The low refractive index layer also includes at least one additive selected from phosphorus and boron to moderate strain in each refractive index layer which is caused by difference in heat expansion ratio between high and low refractive index layers (U.S. Pat. No. 4,701,663). The high refractive index layers includes at least one additive selected from phosphorus, boron, arsenic, antimony, tin, zinc, lead, potassium, nickel, and cobalt and the low refractive index layer includes at least one additive selected from phosphorus and boron (U.S. Ser. No. 925388/86). Therefore, cracks caused by difference in heat expansion ratio between the two layers can be decreased. However, prevention of peeling between high and low refractive index layers of optical interference film is not sufficient. In particular, such a peeling also occurs in a dichroic film attached to a lamp with a reflection mirror operation temperature of which is relatively low. Therefore, it is rather difficult to accumulate each refractive index layer at a desirable numbers.